(1) Field of the Invention
The present invention relates to a fixing device and an image forming apparatus, and particularly to an art of preventing meandering of a fixing belt included in a fixing device employing a resistance heating system.
(2) Related Art
There has been known the structure of an image forming apparatus employing an electronic photography system in which an endless belt-shaped fixing rotary member (hereinafter, fixing belt) is used in order to improve the thermal efficiency for thermal fixing of a toner image carried on a recording sheet. By reducing the thickness of the fixing belt to decrease the thermal capacity thereof, it is possible to reduce the warming-up period and the thermal efficiency during warming-up and fixing. Furthermore, in the case where the fixing belt also functions as a heat source in a fixing device employing an electromagnetic induction heating system, a resistance heating system, and so on, a small thermal loss occurs because a thermal conduction path from the heat source to a recording sheet is extremely short.
In order to thermally fix a toner image onto a recording sheet, a fixing nip needs to be formed by bringing a pressure roller into pressure-contact with the fixing belt. Accordingly, a pressing member such as a fixing roller is brought into pressure-contact with a region on the inner circumferential surface of the fixing belt which corresponds to the fixing nip. In order to reduce a thermal loss resulting from thermal conduction from the fixing belt to the pressing member, it is effective to adopt the structure in which the fixing belt and the pressing member are loosely fit together by providing a space therebetween. Air with high thermal insulation properties enters the space and to exist between the fixing belt and the pressing member, and this effectively reduces the thermal loss resulting from the thermal conduction as described above.
However, according to the above structure in which the fixing belt and the pressing member are loosely fit together, there occurs a problem that the fixing belt meanders in the rotation axis direction thereof to cause belt deflection because of not being tightly held. Especially if the fixing belt continues to deflect in the same direction on the rotation axis thereof, there might occur faulty fixing, damage on the fixing device, drop-off of the fixing device, and so on. Therefore, there has been proposed a countermeasure of providing a meandering prevention member that prevents the fixing belt from meandering so as to face each side of the fixing belt in the width direction of the fixing belt.
For example, Japanese Patent Application Publication No. 2010-249917 has proposed an art with respect to a fixing device employing the electromagnetic induction heating system. According to this art, a pair of meandering prevention members, which are held so as to independently rotatable relative to a fixing roller, are in abutment with only a part of the fixing belt where a fixing nip is not formed when seen in the rotation axis direction of the fixing belt. With this structure, the meandering prevention members are driven by the fixing belt to rotate, differently from the case where the meandering prevention members are fastened together with the fixing roller. This minimizes the difference in peripheral speed between the fixing belt and the meandering prevention members at the abutment position therebetween. Therefore, it is possible to prevent abrasion, shaving, and so on due to sliding contact of the fixing belt with the meandering prevention members.
Also, while the peripheral speed of the fixing belt is constant even at the fixing nip, irrespective of the distance from the rotation axis of the fixing belt, the peripheral speed of the meandering prevention members varies in accordance with the distance of the rotation center thereof. By not bringing the meandering prevention member into abutment with the fixing belt at the fixing nip which is especially close to the rotation axis of the fixing belt, it is possible to further reduce the difference in peripheral speed between the meandering prevention member and the fixing belt at the abutment position therebetween.
According to the fixing device employing the electromagnetic induction heating system, the fixing belt receives an external force only at the fixing nip. Accordingly, the fixing belt has a cross section perpendicular to the rotation axis of the fixing belt that forms a rotation path in the shape of a substantial circle or an ellipse excepting the fixing nip. The fixing belt runs on this rotation path. Also, this circle or ellipse has its center and focal points on a straight line connecting the rotation center of a pressure roller and the rotation center of a fixing roller on the cross section of the fixing belt perpendicular to the rotation axis. This straight line is hereinafter referred to as straight line in the pressure direction.
Compared with this, according to a fixing device employing the resistance heating system, since electric power needs to be fed to a resistance heating layer, an electrode part needs to be provided at each side of a fixing belt in the width direction of the fixing belt to bring a power feeding brush into abutment with the electrode part. In the case where the power feeding brush is in abutment with the electrode part at a position which is not positioned on the straight line in the pressure direction, the fixing belt runs on a rotation path in the shape of a substantial ellipse formed by the cross section perpendicular to the rotation axis. However, a straight line connecting two focal points of this substantial ellipse is not coincident with the straight line in the pressure direction due to a pressing force of the power feeding brush. For this reason, even if the above conventional art is applied with no modification, it is impossible to sufficiently reduce the difference in peripheral speed between the fixing belt and meandering prevention members.
According to the fixing device employing the resistance heating system, however, when the fixing belt meanders, the electrode part of the fixing belt is brought into pressure contact and sliding contact with the meandering prevention member, and the electrode part deforms to uplift. This causes instantaneous defective continuity at a part of the electrode part that in abutment with the power feeding brush, and results in a large difference in electrical potential to cause a spark discharge. The electrode part melts or is damaged due to heat and shock of the spark discharge, and as a result the outer surface of the electrode part becomes uneven. Since the surface flatness of the electrode part is not uniform in this way, the sliding contact state between the power feeding brush and the electrode part becomes destabilized. This hinders a stable power feeding to the electrode part and thereby the resistance heating layer. Furthermore, a spark discharge frequently occurs, and accordingly the durability of the electrode part deteriorates and thereby the life time of the fixing belt extremely decreases. Therefore, even in the fixing device employing the resistance heating system, prevention of meandering of the fixing belt is a problem absolutely to be solved.